If
your experience with the whole PC scene is limited
to the past twelve months then you'd be forgiven for
not ever having heard the term "lapping"
before. Although it seems to have faded a little in
popularity, it was once one of those geeky pursuits
that ranked alongside other mysterious black arts
like overclocking and water cooling, and anyone who
was anyone had lapped a heatsink and lived to tell
about it.

Lapping,
in simple terms, is the flattening, smoothing and
finishing of a metal surface. It can be done by machine
or by hand and almost always involves motion and some
form of abrasive. But why?

Well,
let's imagine the diagram below represents a highly
magnified cross section of how the base of your heat
sink looks when it's sat on your CPU core or the heat
spreader that covers it.

Because
neither surface is perfectly flat there are a few
contact areas but there are also a lot of places where
neither of the surfaces actually meet. As I'm sure
you can imagine, heat from the core travels easily
across the contact areas but, due to the relatively
low thermal conductivity of air, it has a much tougher
job bridging the areas where there is no contact.

Unlapped
~ No Thermal Grease

This
is the reason why thermal compounds are so important
when fitting a heat sink. The thermal compound has
a much higher thermal conductivity than air and so
helps heat to bridge the gaps where air would normally
reside. The thermal compound filled areas still aren't
as efficient at shipping heat as those in physical
contact but the solution is still vastly preferable
to dry mating of the surfaces.

Unlapped
~ With Thermal Grease

There
is however a further step that can be taken to improve
thermal transmission and that is to abrade away (or
lap) the imperfections in one or, ideally, both mating
surfaces.

As
you can see, with just the heat sink base lapped,
there is an increased amount of contact and a reduced
volume of thermal compound. If you ever wondered why
it'd important not to use too thick a layer of thermal
grease, this is why. You want just enough to fill
the surface pits and not so much that it forms a cushioning
layer that keeps the two surfaces apart.

HSF
Lapped ~ With Thermal Grease

Naturally,
the progression to this is to lap both mating surfaces
which, if done properly, would virtually eliminate
the need for any kind of thermal interface between
the two. That said I think I'd still use a thin smear
of a very fine silver or ceramic based compound which
may well still do some good on a microscopic level.

HSF
and Core Lapped

So
that's what lapping is all about, but who should do
it and how?

Well
the who is easy. If you've got the time and the patience
and you're not expecting to see miraculous temperature
gains then lapping is suitable for just about anyone.

As
for the how, well, you can buy all the parts you need
quite easily at any half decent hardware store, but
you need a pretty good idea of exactly what you want,
and you need to be ready to be sold far more of each
component than you actually need.

If
that idea doesn't grab you, then you could take the
vastly easier route and head over to EasyPCKits
to order one of their lapping kits that comes complete
with everything you need including detailed instructions,
the full range of grits, glass (if required) and a
tube of quality thermal compound.

The
glass is a cheap but effective way to provide a perfectly
flat surface on which to work, but more on that in
a moment.

For
this review I'll be trying out the premium kit which
contains one each of the following 1/4 sheets of sandpaper:

400

600

800

1000

40
micron (yellow)

25
micron (green)

20
micron (blue)

15
micron (peach)

10
micron (light green)

It
also came with a piece of glass and a tube of Arctic
Silver Ceramique thermal compound.